Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling taken at 'callq' instruction

[Peter Zijlstra]

On Tue, Dec 01, 2015 at 08:28:26AM +0100, Ingo Molnar wrote:
> > Not sure what you're getting at here. We don't need the uncorrected
> > instruction.
> 
> Well, we need it for stack unwinding, as you point it out:
> 
> > But the problem here is that we rewind the instruction stream, but not
> > the stack. And the stack unwinder is (obviously) interested in the stack
> > state.
> 
> Unwinding the stack state would fix it as well - but an equivalent solution would 
> be to pass along the original RIP would fix it as well: we'd have a 
> self-consistent pair of RIP/RSP.
> 
> Especially since unwinding the RSP is probably hard:
> 
> > I'm not sure we want (or need) to go undo the specific instruction's
> > stack effect in-kernel. If the !DWARF unwinders are similarly confused
> > we might need to put it in kernel (expensive *groan*). If its only the
> > DWARF muck then its something that can be done in userspace just
> > fine, although we might need to copy slightly more of the stack than SP
> > is pointing at, such that we can undo RET/POP etc. which would have data
> > beyond the head of stack.
> > 
> > The easiest solution might be to figure out the biggest stack offset for
> > any instruction and always capture that much over the head of stack.
> 
> so I think the problem here is that the RSP does not match up to the RIP. We can 
> either pass along the original RIP+RSP, or the fixed up one - but what we do 
> currently is that we pass along only half of it - which corrupts dwarf unwinding 
> state that doesn't tolerate such errors.

Still not sure what that gets you. Then you get a sample at a known
wrong location, why would you want that?
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